Thyroid cancer is the most common endocrine malignancy. A majority of patients have differentiated thyroid cancer (DTC) and are managed successfully with a combination of surgery, radioiodine and thyroid hormone replacement therapy. Anaplastic thyroid cancer (ATC), on the other hand, is one of the most aggressive human malignancies with an average survival of only 4 to 6 months. There are currently no effective therapies for ATC indicating that we need better tools to understand and treat this aggressive disease. TXNIP is a potent tumor suppressor, plays an important role in oxidative stress and is a major regulator of glucose uptake in cells. This protein has not been studied in normal thyrocytes or thyroid cancer. We have, for the first time, shown that TXNIP is expressed in normal thyroid cells and differentiated thyroid cancer, but is undetectable or expressed at low levels in poorly differentiated and undifferentiated (ATC) thyroid cancer and cells lines. We believe that this tumor suppressor and regulator of glucose uptake is responsible for the indolent behavior of many well-differentiated thyroid cancers and may explain why some patients with PET negative (low glucose uptake) metastatic disease do well, while most patients with PET positive (high glucose uptake) metastatic disease have a much worse prognosis. In this proposal, we will genetically manipulate TXNIP levels in normal thyroid cells as well as differentiated and undifferentiated thyroid cancer cells to assess the role of this potential tumor suppressor in cancer progression. We have also treated a panel of ATC cell lines with pharmacologic inhibitors of important signaling pathways in thyroid cancer and made the novel observation that inhibitors of the PI3K-mTOR pathway induce re-expression of TXNIP in these ATC cells. We will test these genetic and pharmacologic approaches to TXNIP regulation in vivo using a clinically-relevant orthotopic tumor model where thyroid cancer cells are injected directly into the thyroid glands of mice. We will also utilize a metastatic model of direct intracardiac injection of cancer cells, which leads to lung, bone and brain metastases. This model was developed by our research group and is completely new to the thyroid cancer field. Finally we will develop novel transgenic mouse lines which will genetically delete or overexpress TXNIP in the thyroid cells. Successful completion of the aims in this proposal will advance our understanding of the role of TXNIP in normal thyrocyte development and function, susceptibility to development of thyroid cancer, as well as advanced thyroid cancer tumor progression, dedifferentiation, metastases and metabolism. Ultimately, these studies will provide new tools to understand thyroid physiology and cancer pathophysiology, and will lead to new tumor markers and therapeutic targets to improve treatment and monitoring of patients with advanced thyroid cancer.